One type of a conventional circuit for driving an inductive load comprises a switching transistor, to which an input signal is applied.
In operation, the input signal is applied to the switching transistor, thereby being switched on, so that a driving voltage is applied across the inductive load. Thus, the inductive load is driven, so that a driving current flows through the inductive load. In this driving circuit, there is an advantage in that less power consumption is realized, because D-class amplification is carried out in the switching transistor.
However, this conventional circuit for driving an inductive circuit has a disadvantage in that the driving current includes impulse noise which is resulted from the differential of the driving voltage. The impulse noise includes harmonic components.
A motor is used for driving a magnetic recording medium in a magnetic recording and reproducing apparatus such as a tape recorder, a VTR, a magnetic disc, and the like. In such a use, the motor is turned on and off, is changed in its polarity, and is controlled in its velocity by use of a pulse width modulation (PWM). In such a case, the aforementioned impulse noise is liable to be produced in a driving current at the time of the turning on and off of the motor. The impulse noise is introduced into a magnetic head, a wiring of an input circuit, etc. in the magnetic recording and reproducing apparatus, so that the impulse noise appears as harmonic noise at a frequency band used therein at the time of reproducing information recorded in the magnetic recording medium, thereby decreasing a S/N ratio.
For avoiding this disadvantage, a capacitor is connected in parallel with the motor to decrease the harmonic noise. However, this is not sufficient to overcome this disadvantage, because the provision of the capacitor is no more than one-order low pass filter.
In this circumstance, another type of a circuit for driving an inductive load is used, for instance, in the magnetic recording and reproducing apparatus. The circuit for driving an inductive load comprises a low pass filter for relaxing a steep rise and a steep fall of an input signal, and a power amplifier for amplifying the input signal thus relaxed at the rise and the fall thereof.
In operation, the input signal is supplied to the low pass filter, so that the input signal is relaxed at the rise and the fall thereof. Then, the relaxed input signal is amplified in the power amplifier to produce an amplified driving voltage which is applied across the motor, so that the motor is turned on and off to drive the magnetic recording medium. In this circuit for driving an inductive load, the amplified driving voltage rises up with a relaxed rise, so that a driving current flowing through the motor in the application of the driving voltage is suppressed in its peak value, because the driving current is defined by differentiating the driving voltage. Consequently, harmonic components are decreased in the driving current, and a S/N ratio is increased.
However, the latter type of the conventional circuit for driving an inductive load has disadvantages in that it is expensive due to the inclusion of the power amplifier, and a power consumption is increased, because the power amplifier is switched on and off in analog operation with the modulated rise and fall of the input signal.